This invention relates to a method and apparatus for thermally processing flowable materials while utilizing a radiant heater system. The invention relates particularly to a unique system which is well suited for thermally processing powders, pellets, wet filter cakes, slurries and sludges which exhibit poor infrared absorptivity and which cannot be thermally treated efficiently when exposed directly to sources of infrared radiation.
The invention comprises a system which may be implemented utilizing components of an apparatus of the type generally described in U.S. Pat. No. 5,634,282. This apparatus consists of an elongated housing having an axially mounted rotatable shaft disposed therein. A plurality of paddles or vanes are mounted on the rotatable shaft and these extend outwardly from the periphery of the shaft. In accordance with known practices, the paddles are set at an angle whereby rotation of the shaft operates to continuously move material from the inlet end of the unit toward the outlet end. As described in the aforementioned patent, the apparatus is provided with spaced apart infrared radiant heaters which are mounted over the openings in the top wall of the housing. By introducing flowable material at one end of the vessel, treatment of the material is achieved by means of radiant energy of electric or gas operated radiant heaters or by means of electromagnetic waves of the electromagnetic spectrum which produces heat upon being absorbed by the material being processed.
As further described in the aforementioned patent, the radiant heater system has certain distinct advantages over prior art arrangements. By relying primarily on radiant heating rather than on conductive or convective heating, dependence on the intervening medium such as heated air to achieve a desired temperature is avoided. Due to direct heat transfer to the particles by means of radiant energy, the material temperature can be maintained efficiently at an optimal level. The heat transfer rate with infrared radiation is much higher than with convective systems (such as a fluid bed) or with a conductive system (such as jacketed indirect heat supply thermal processors). As illustrated in the patent, with such higher heat transfer rates, more thermoprocessing can be accomplished in less space. The infrared radiant heaters have low thermal mass (inertia) and can, therefore, respond almost instantaneously to modulating controls. Accordingly, the temperature of material being processed can be maintained precisely.
A system of the type contemplated is especially suited for high temperature thermal treatment of flowable materials when other known methods of heat supply, such as one based, for example, on circulation of hot liquid medium through a jacketed processor, are unusable due to their temperature limit. It is recognized that the temperature limit for liquid heating media available in the industry is 700-750.degree. F. while the infrared radiant heaters (for instance, the high density infrared heater of a type manufactured by Research Inc., Model 5208) can heat material being processed up to 2,500.degree. F.
It has, however, also been recognized that available infrared heating processors are inefficient when treating some materials such as fine powders, white or light-colored materials with low radiative absorptivity, and difficult to handle heavy viscous materials (filter cakes, slurries and sludges) which exhibit thixotropic characteristics when subjected to agitation and shear forces. Fine powders, being mechanically fluidized in an agitated processor, tend to generate an airborn aerosol which reduces the heat transfer efficiency of the radiative flux and which can cause an undesirable deposition of dust particles on the emitting surfaces of the radiant heater, resulting in failure of the processor. White and light-colored flowable materials tend to reflect the radiation received (their absorptivity is very low), thereby increasing the required capital equipment cost and operating expenses.
There has been a long-recognized difficulty in utilization of the prior art infrared systems for thermal processing of shear-sensitive viscous materials. When shear force is applied during thermal processing and conveying, these materials, due to their thixotropic characteristics, revert from a free flowing cake or sludge to a heavy, viscous hard-to-handle paste. In drying applications, the viscosity of such material is constantly increasing as water (or solvent) are evaporated, and the infrared processor then gradually becomes less efficient or incapacitated.
It has also been recognized that available infrared heater systems cannot be used for processing some flammable materials (containing, for example, flammable solvents), as well as for treatment of some heat-sensitive organic materials and chemicals which can decompose or change their quality (color, for example) when being exposed to direct infrared radiation.